Storage

Providing surface storage

When providing effective surface flood pathways for extreme events in existing urban areas one of the challenges faced is that the space may not be available to achieve the required conveyance capacity. Where full surface conveyance capacity cannot economically be provided, reduced capacity may be accommodated if flows can be attenuated on site. This can be achieved by the planned provision of surface storage.

As with surface flood channels, surface storage can be accommodated using areas that are used for other purposes for most of the time. When considering potential areas the following questions should be answered:

1. What depth of storage would be necessary to achieve the required flooding volume?

2. How long will it take for the area to drain after the event?

3. How will the temporary storage of flood volume affect the primary use of the area? Is flood warning necessary? If so, what is the process

4. Will any damage or important loss of use occur?

5. Will standing water create an unacceptable risk to public health or safety?

Storage on an existing car park to a moderate depth less than kerb height might be acceptable (Figure 1). Structural damage is likely to be insignificant, and if only surface runoff is being stored, then health and safety risks are likely to be acceptable. There will be some loss of use, but provided the area drains relatively quickly after the event, this should not be a problem in most situations.

The required storage volume can in many cases be large. This is especially true where storage is being used to mitigate the impacts of exceedance flow conveyance to downstream systems. The designer should consider the potential of sacrificial areas in such cases. These are areas of low value land to which exceedance flood volumes can be discharged and retained for longer periods of time. Such areas may not have identified outfalls with the water stored infiltrating slowly into the ground and/or evaporating over a period of time after the event.